1. Field of the Invention
The present invention relates to a vehicle-installed fuel cell system and a fuel cell control method.
2. Description of Related Art
Recently, fuel cell systems have drawn attention as new sources of power that can be used to drive vehicles. For example, a fuel cell system can be configured with a fuel cell which produces electric power from chemical reactions of reactive gases, wherein a reactive gas supplying unit supplies reactive gases to the fuel cell via a reactive gas channel, and a control unit controls the reactive gas supplying unit.
In another example, a fuel cell can be structured to include a plurality, e.g., tens or hundreds, of stacked cells. In such an example, each cell is configured with a membrane electrode assembly (MEA) sandwiched between a pair of separators. The MEA is configured with two electrodes, that is an anode (i.e., a positive electrode) and a cathode (i.e., a negative electrode), and a solid polymer electrolyte membrane sandwiched between the two electrodes.
Supply of hydrogen gas and oxygenated air as reactive gases to the anode and cathode of the fuel cell, respectively, causes an electrochemical reaction from which the fuel cell produces electric power. Since only water, which is essentially harmless to the environment, is generated during power production, the fuel cell has garnered attention from the viewpoint of environmental impact, and availability of the technology.
When the aforementioned fuel cell system is activated at a low temperature, a phenomenon wherein the generated water condenses within the fuel cell (flooding) may occur.
To solve the aforementioned problem, a method of supplying oxidized gas to the fuel cell at an excessively higher flow rate than the standard rate until the fuel cell warms up by self-heating when the system is activated at a low temperature has been proposed in Japanese Unexamined Patent Application Publication No. 2005-116257 (hereafter referred to as JP '257).
The fuel cell system disclosed in JP '257 discharges the majority of generated water from the fuel cell by supplying an excessive amount of oxidized gas, which prevents any flooding of the fuel cell, and facilitates stable activation of the system at a low temperature.
Although antifreeze, which does not freeze at 0° C., is contained in pores within the MEA, the antifreeze may freeze in a cryogenic environment, such as at approximately 40° C. below freezing. In such a case, pores between a reactive gas supply channel and the inside of the MEA are closed due to ice. As a result, the supply of reactive gases cannot reach a reactive area (i.e., catalyst surface) within the MEA.
Frozen antifreeze behaves the same as regular ice. Accordingly, the frozen antifreeze should be melted by warming up the interior of the MEA until the temperature within the MEA exceeds 0° C. in order to reopen the gas supply channel to the reactive area.
However, even if the fuel cell system of JP '257 is intended to be activated below freezing (below 0° C.), when the fuel cell is exposed to a cryogenic environment and the antifreeze within the MEA freezes, an area within the MEA where ice is formed is not able to produce power. As a result, there is a decrease in total areas within the MEA that is available to produce power.
As mentioned above, the current density in the areas within the MEA that are available to produce power increases locally because the fuel cell produces enough electric power for the required output voltage even though the antifreeze within the MEA has frozen. As a result, a partial current density of the MEA becomes excessively high, resulting in the accelerated deterioration of the MEA in that area.